The demand for increasing the packing density of IC packages, multilayer wiring boards and other products of microelectronics is constantly growing and this has caused the need to develop commercially feasible low-dielectric constant ceramics and low-resistance conductors. Metals that have been commonly employed to make metallizing compositions are noble metals such as Au, Au-Pt, Ag-Pt and Ag-Pd, as well as high-melting point base metals such as W, Mo and Mo-Mn. Paste of noble metals such as Au, Au-Pt, Ag-Pt, and Ag-Pd have the advantage that it can be fired in air but, on the other hand, it suffers from the problem of high cost. The high-melting point metals such as W, Mo, and Mo-Mn have the advantage that they can be easily formed in multiple layers since they can be fired simultaneously with ceramics at about 1,600.degree. C. or the temperature at which green ceramic sheets are sintered. Problems with these high melting point metals are low electroconductivity, potential hazard in operations that require firing in a reducing atmosphere, and the need to plate the conductor surface with Ni and other suitable metals to enable soldering. Under these circumstances, use has increased of ceramic wiring boards that employ Cu paste that is inexpensive and which offers high electroconductivity and good solderability.
A problem with this Cu paste is that in the manufacture of ceramic wiring boards, it is difficult to completely combust the organic matter in the paste without causing oxidation of Cu. If the Cu surface is oxidized, not only is the solderability of the paste impaired but its electroconductivity is also reduced. If care is taken to avoid the generation of CuO, the organic binder and other materials will remain without being completely burned away.
Another problem with the use of metallic Cu is that even if the step of removing the binder is separated from the step of Cu firing, the metallic Cu is oxidized to CuO in the step of binder removal and this causes an expansion in volume, leading to various problems such as separation from the substrate.
The Cu paste presents other problems when using it in the fabrication of multilayer wiring boards; the need to perform firing each time after printing and drying operations have been completed results in a prolonged lead time, as well as an increase in the initial cost.
With a view to solving these problems, various proposals have been made; JP-A-61-288484 (the term "JPA" as used hereinafter means an unexamined published Japanese patent application) discloses a metallizing composition comprising CuO, MnO, a binder, a plasticizer and an organic solvent; JP-A-61-292393 discloses a metallizing composition comprising CuO, an inorganic solid component, a binder, a plasticizer and an organic solvent; and JP-A-61-292394 discloses a metallizing composition comprising at least one metallic component selected from the group consisting of CuO, MnO, Pt, Pd and Ni, a vehicle and a solvent. However, none of these proposals is particularly directed to metallic Cu.
The present inventors previously proposed in Japanese Patent Application No. 62-129441 a metallizing composition based on CuO and Cu that was particularly intended for use in a high-density multilayered ceramic wiring board and with which through-holes were to be filled to ensure satisfactory current conduction by a conductor. They also described in Japanese Patent Application No. 62-122486 that a metallizing composition based on Cu and CuO is suitable for use in the manufacture of high-density multilayered wiring boards since it produces an air-tight and strong adhesion to the ceramics and crystallizable glass in the substrate.